In recent years, an electronic key system (also called a smart entry system, etc.) has spread. In this electronic key system, ID authentication is performed by wireless communication between this system and a wireless electronic key (also called a portable device) carried by a user. Further, controls of locking/unlocking of a door lock, engine starting, etc. can be performed by commands from this portable device. In the above wireless electronic key, a demand for constructing this wireless electronic key as a thin card type wireless device made thin to improve carrying convenience for storing this wireless electronic key into a purse, etc. with a dramatic increased use of an IC card, etc. as background (3 mm or more and 5 mm or less in thickness).
The above electronic key system adopts a communication system able to execute a control operation such as locking/unlocking of the door lock and engine starting if the user approaches the automobile within a predetermined distance even when no user performs a special button operation, etc. with respect to the wireless electronic key. For example, a request radio wave sent out of the automobile side in one direction is received. ID authentication information, control command information relating to the above locking/unlocking or the engine starting, etc. are superposed on the transmitted radio wave and are sent out to the automobile side. In this ease, when the user is distantly located, the wireless electronic key and the automobile do not react on communication. On the other hand, when the user approaches, there are many cases in which near distance type direct communication using a low frequency band (50 kHz or more and 500 KHz or less) is adopted so as to detect the radio wave by detouring the radio wave even when the user holds the wireless electronic key in any portion of the user's body.
The radio wave of the low frequency band has a very long wavelength. Therefore, in an antenna used for this radio wave, a so-called LF (Low Frequency) antenna provided by combining an antenna coil and a capacitor resonantly coupled to this antenna coil in a desirable frequency band is normally adopted. When the LF antenna is assembled into the card type wireless device, it is also necessary to reduce the thickness of this antenna coil in conformity with the thickness of a box body of the card type (e.g., 1 mm or more and 3 mm or less). In this case, it is desirable to mount the antenna coil onto a substrate in a shape for largely setting the aperture diameter of the antenna coil as much as possible to raise sensitivity with respect to the radio wave perpendicularly incident to the substrate face. It is effective to adopt the antenna coil with a core of high inductance so as to raise antenna gain. However, a flat ferrite core is small in mechanical strength, and a crack, a fragment, etc. is easily caused by handling, etc. at coil winding time. Accordingly, an air-core coil is normally adopted.
However, since the coil of an air-core type may have no physical support core, there are defects in that no radio wave magnetic field can be concentrated onto the circumference of the coil and sensitivity and antenna gain are inferior.